🧪 Formulae, Equations & Chemical Reactions
Secondary School Chemistry • MalawiHub
1. Introduction
Chemical formulae show the elements and relative numbers of atoms in a substance. Chemical equations describe chemical reactions using symbols and formulae.
Objectives
- Write chemical formulae correctly.
- Write and balance chemical equations.
- Identify reactants and products.
- Recognise common types of chemical reactions.
- Use state symbols and reaction conditions.
2. Writing Chemical Formulae
The formula of a compound must have an overall electrical charge of zero.
| Ion | Charge |
|---|---|
| Na⁺ | +1 |
| Mg²⁺ | +2 |
| Al³⁺ | +3 |
| Cl⁻ | −1 |
| O²⁻ | −2 |
| SO₄²⁻ | −2 |
Mg²⁺ requires two Cl⁻ ions.
Formula = MgCl₂
3. Chemical Equations
A chemical equation represents a reaction. The substances present before the reaction are reactants; the substances formed are products.
Magnesium + oxygen → magnesium oxide
2Mg + O₂ → 2MgO
State Symbols
(s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous solution.
Example:
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
4. Balancing Equations
Equations are balanced because atoms are neither created nor destroyed during a chemical reaction.
Unbalanced: H₂ + O₂ → H₂O
Balanced: 2H₂ + O₂ → 2H₂O
Important: Never change the small numbers inside chemical formulae when balancing. Change only the coefficients in front.
5. Types of Chemical Reactions
| Type | Example |
|---|---|
| Combination | 2Mg + O₂ → 2MgO |
| Decomposition | CaCO₃ → CaO + CO₂ |
| Displacement | Zn + CuSO₄ → ZnSO₄ + Cu |
| Neutralisation | HCl + NaOH → NaCl + H₂O |
| Combustion | CH₄ + 2O₂ → CO₂ + 2H₂O |
6. Ionic Equations
Ionic equations show only the particles that actually take part in a reaction.
HCl + NaOH → NaCl + H₂O
Net ionic equation:
H⁺(aq) + OH⁻(aq) → H₂O(l)
7. Reaction Conditions
Some reactions require special conditions such as heating, light, electricity or a catalyst.
- Δ means heating.
- A catalyst speeds up a reaction without being permanently consumed.
- Electricity can cause electrolysis.
- Light can initiate some reactions.
8. MANEB Examination Focus
- Write correct formulae from ions.
- Balance equations.
- Identify reactants and products.
- Use correct state symbols.
- Identify reaction types.
- Write simple ionic equations.
- State conditions required for reactions.
Balance the equation:
Al + O₂ → Al₂O₃
4Al + 3O₂ → 2Al₂O₃
9. Practice Exercises
- Write the formula of aluminium oxide.
- Write the formula of calcium chloride.
- Balance: Fe + O₂ → Fe₂O₃.
- Balance: Na + H₂O → NaOH + H₂.
- Identify the reaction type: Zn + CuSO₄ → ZnSO₄ + Cu.
- What does (aq) mean?
- Write the ionic equation for neutralisation.
Answers
- Al₂O₃
- CaCl₂
- 4Fe + 3O₂ → 2Fe₂O₃
- 2Na + 2H₂O → 2NaOH + H₂
- Displacement.
- Aqueous solution.
- H⁺ + OH⁻ → H₂O
10. Common Mistakes
- Do not change subscripts when balancing.
- Check that every element has equal numbers of atoms on both sides.
- Use correct ion charges when writing formulae.
- Do not confuse coefficients with subscripts.
11. Exam Tip
After balancing an equation, count every type of atom on both sides. This simple check prevents many examination errors.